package com.lzx.hbh_system.util;

public class SnowFlakeUtil {
        private long workerId;
        private long datacenterId;
        private long sequence = 0L;
        private long twepoch = 1288834974657L;                              //  Thu, 04 Nov 2010 01:42:54 GMT 标记时间 用来计算偏移量，距离当前时间不同，得到的数据的位数也不同
        private long workerIdBits = 5L;                                     //  物理节点ID长度
        private long datacenterIdBits = 5L;                                 //  数据中心ID长度
        private long maxWorkerId = -1L ^ (-1L << workerIdBits);             //  最大支持机器节点数0~31，一共32个
        private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);     //  最大支持数据中心节点数0~31，一共32个
        private long sequenceBits = 12L;                                    //  序列号12位， 4095，同毫秒内生成不同id的最大个数
        private long workerIdShift = sequenceBits;                          //  机器节点左移12位
        private long datacenterIdShift = sequenceBits + workerIdBits;       //  数据中心节点左移17位
        private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; //  时间毫秒数左移22位
        private long sequenceMask = -1L ^ (-1L << sequenceBits);                          // 用于和当前时间戳做比较，以获取最新时间
        private long lastTimestamp = -1L;

        //成员类，SnowFlakeUtil的实例对象的保存域
        private static class IdGenHolder {
            private static final SnowFlakeUtil instance = new SnowFlakeUtil();
        }
        //外部调用获取SnowFlakeUtil的实例对象，确保不可变
        public static SnowFlakeUtil get(){
            return IdGenHolder.instance;
        }
        //初始化构造，无参构造有参函数，默认节点都是0
        public SnowFlakeUtil() {
            this(0L, 0L);
        }
        //设置机器节点和数据中心节点数，都是 0-31
        public SnowFlakeUtil(long workerId, long datacenterId) {
            if (workerId > maxWorkerId || workerId < 0) {
                throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
            }
            if (datacenterId > maxDatacenterId || datacenterId < 0) {
                throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
            }
            this.workerId = workerId;
            this.datacenterId = datacenterId;
        }

        //线程安全的id生成方法
        @SuppressWarnings("all")
        public synchronized long nextId() {
            //获取当前毫秒数
            long timestamp = timeGen();
            //如果服务器时间有问题(时钟后退) 报错。
            if (timestamp < lastTimestamp) {
                throw new RuntimeException(String.format(
                        "Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
            }
            //如果上次生成时间和当前时间相同,在同一毫秒内
            if (lastTimestamp == timestamp) {
                //sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
                sequence = (sequence + 1) & sequenceMask;
                //判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
                if (sequence == 0) {
                    //自旋等待到下一毫秒
                    timestamp = tilNextMillis(lastTimestamp);
                }
            } else {
                //如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加，每个毫秒时间内，都是从0开始计数，最大4095
                sequence = 0L;
            }
            lastTimestamp = timestamp;
            // 最后按照规则拼出ID 64位
            // 000000000000000000000000000000000000000000  00000            00000       000000000000
            //1位固定整数   time                                       datacenterId   workerId    sequence
            return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
                    | (workerId << workerIdShift) | sequence;
        }

        //比较当前时间和过去时间，防止时钟回退（机器问题），保证给的都是最新时间/最大时间
        protected long tilNextMillis(long lastTimestamp) {
            long timestamp = timeGen();
            while (timestamp <= lastTimestamp) {
                timestamp = timeGen();
            }
            return timestamp;
        }

        //获取当前的时间戳（毫秒）
        protected long timeGen() {
            return System.currentTimeMillis();
        }

        /**
         * 获取全局唯一编码
         */
        public static String getId(){
            Long id = SnowFlakeUtil.get().nextId();
            return id.toString();
        }

        public static void main(String[] args) {
            System.out.println(get());
        }
}
